Cell-cell signaling controls many processes in the biological world, including development, pathogenesis, growth, transformation, and mating. Signaling processes are often mediated by diffusible factors (e.g., hormones, pheromones, neurotransmitters) that are produced by some cells and sensed by others. In addition to diffusible signaling molecules, one of the most intimate forms of cell-cell communication involves conjugation and horizontal gene transfer. Mating responses in microbes are often mediated by peptide pheromones. The long term goal of this project is to understand how cells modulate gene expression and horizontal gene transfer in response to environmental conditions, with emphasis on cell-cell signaling and the regulatory mechanisms controlling a mobile genetic element in the bacterium Bacillus subtilis. Mobile genetic elements include phages, conjugative plasmids, and conjugative transposons, also known as integrative and conjugative elements (ICEs). Mobile genetic elements play significant roles in evolution, genome plasticity, symbiosis, pathogenesis, and the spread of antibiotic resistance. Despite their importance, relatively little is known about the mechanisms regulating ICEs in Gram-positive organisms. The mobile genetic element ICEBs1 is an integrative and conjugative element in Bacillus subtilis. It resides in the B. subtilis chromosome and is stably propagated with the host chromosome. ICEBs1 gene expression, excision, replication, and mating are regulated by a cell-cell signaling response and induced upon DNA damage (the SOS response). This project will focus on mechanisms of ICEBs1 regulation and function that appear to be broadly conserved and that are not well-understood in Gram-positives. The project includes: elucidation of the mechanisms of induction mediated by a conserved anti-repressor and repressor and their regulation by DNA damage and cell-cell signaling; determination of the mechanisms and functions of replication of ICEBs1 in donor cells after excision and how replication contributes to horizontal gene transfer and stability of ICEBs1; determination of roles of conserved cell-cell signaling modules in the control of horizontal gene transfer in other mobile elements. Our studies on cell-cell signaling, gene expression, and horizontal gene transfer in B. subtilis, a relatively simple, experimentally accessible microbe, should provide insights into general mechanisms used in many organisms.